The advantageous mechanical properties, dimensional stability and optical properties of biaxially oriented polyester films are well-known. These properties have led to the use of polyester films in electronic and opto-electronic devices including electroluminescent (EL) display devices (particularly organic light emitting display (OLED) devices), electrophoretic displays (e-paper), photovoltaic cells and semiconductor devices (such as organic field effect transistors, thin film transistors and integrated circuits generally). For these and other applications, it is sometimes necessary to provide a very smooth and flat surface for the further processing of the polyester film, for instance to ensure the integrity of subsequently applied coatings, such as a conductive coating or a barrier coating, in order to avoid breaks or pin-pricks therein. In the manufacture of flexible electronic or opto-electronic display devices, for instance, a conductive layer such as indium tin oxide (ITO) may be disposed on a film substrate via a sputtering technique, and non-uniformity in the substrate surface can cause non-uniformity and/or discontinuities in the conductive layer, resulting in for example non-uniform conductivity or pixel yield problems, depending on the type of electronic device.
It is known to reduce defects in layers subsequently applied to a substrate by the provision of an intermediate planarising layer, as taught in WO-03/087247-A for instance. An alternative approach is to provide strippable sacrificial protective layers which are easily peelable from a substrate surface, in order to protect that surface from damage, contamination and/or debris during storage or transport. The sacrificial layers are then stripped from the substrate to leave a clean surface immediately prior to the further processing or installation of the substrate. For instance, WO-2009/105427-A discloses a composite optical compensation film comprising a polyolefin substrate and strippable layers comprising from a variety of polymeric materials including other polyolefins, polyesters, and ionomers. EP-0345884-A discloses a coextruded strippable polyolefin or polyamide film on the surface of a polycarbonate sheet to avoid loss of a volatile UV stabiliser in the polycarbonate, to protect the production equipment from exuded amounts of the stabiliser, and to protect the polycarbonate surface during handling and shipping. EP-0663867-A teaches the provision of strippable sacrificial layers to a multilayer polymeric body by coating and lamination techniques. Other disclosures of strippable skin layers include U.S. Pat. No. 4,540,623 and U.S. Pat. No. 7,396,632. The prior art, however, is primarily focussed on the need to protect a substrate surface from extrinsic debris or physical damage during storage or transport.
It would be desirable to provide a polyester film wherein one or both surfaces thereof exhibit minimal or no surface defects (low intrinsic process defects), and high surface cleanliness (i.e. low extrinsic debris).